1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of controlling a transmit power in a wireless communication system.
2. Related Art
Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future.
To exchange data between a base station (BS) and a user equipment (UE) in the wireless communication system, there is a need to control a transmit power for a transmit signal. In particular, transmit power control of an uplink channel is important in terms of power consumption of the UE and service reliability. In uplink transmission, if a transmit power is too weak, the BS cannot receive a transmit signal of the UE. On the contrary, if the transmit power is too strong, the transmit signal may act as interference to a transmit signal of another UE, and may increase battery consumption of the UE.
A transmit power control (TPC) command is generally used to control a transmit power of a channel between the BS and the UE. In the conventional WCDMA system, the TPC command is used in both uplink and downlink transmission. A dedicated physical uplink channel (DPCCH) is an uplink channel for carrying a TPC command for a downlink channel. A dedicated physical channel (DPCH) is a downlink channel for carrying a TCP command for an uplink channel. A structure of the DPCH may be found in section 5.3.2 of the 3GPP TS 25.211 V7.0.0 (2006-03) “Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 7)”. The DPCCH and the DPCH are dedicated channels used only between a UE and a BS.
An evolved-UMTS terrestrial radio access network (E-UTRAN) system based on orthogonal frequency division multiple access (OFDMA) is currently being standardized. The E-UTRAN system is also referred to as a long term evolution (LTE) system. Examples of downlink physical channels used in the E-UTRAN include a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH). Unlike the WCDMA system, the PDCCH is only one physical control channel in the E-UTRAN. The PDCCH can carry dedicated control information for a UE or common control information for all UEs in a cell. Examples of uplink physical channels include a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH).
In general, in order for a BS to transmit a TPC command to a UE, when the BS reports downlink resource allocation information and/or uplink resource allocation information to the UE, the TPC command is transmitted on the PDCCH together with the reported information. The downlink resource allocation information is transmitted when there is data to be transmitted by the BS to the UE. The uplink resource allocation information is transmitted after the UE requests the BS to allocate resources. However, the TPC command cannot be transmitted to the UE when neither the downlink resource allocation information nor the uplink resource allocation information is transmitted. For example, when the UE transmits or receives a voice over Internet protocol (VoIP) packet by using radio resources pre-allocated through semi-persistent scheduling, the UE may not be able to receive the TPC command since resource allocation information is unnecessary.
When only the TPC command is transmitted for power control to the UE through one type of PDCCH, radio resources may be ineffectively used since a size of the PDCCH is significantly large while a size of the TPC command is only several bits.
Accordingly, there is a need for a method capable of effectively transmitting a TPC command to a UE through a PDCCH.